Gyros have been used to measure rotation rates or changes in angular velocity about an axis. A basic conventional fiber optic gyro (FOG) includes a light source, a beam generating device, and a coil of optical fiber coupled to the beam generating device that encircles an area. The beam generating device transmits light beams into the coil that propagate in a clockwise (CW) direction and a counter-clockwise (CCW) direction along the core of the optical fiber. Many FOGs utilize glass-based optical fibers that conduct light along a solid glass core of the fiber. The two counter-propagating (e.g., CW and CCW) beams experience different pathlengths while propagating around a rotating path, and the difference in the two pathlengths is proportional to the rotational rate.
In a resonator fiber optic gyro (RFOG), the counter-propagating light beams are desirably monochromatic (e.g., in a single frequency) and circulate through multiple turns of the fiber optic coil and for multiple passes through the coil using a recirculating device such as a fiber coupler. The beam generating device modulates and/or shifts the frequencies of each of the counter-propagating light beams so that the resonance frequencies of the resonant coil may be observed. The resonance frequencies for each of the CW and CCW paths through the coil are based on constructive interference of successively circulated beams in each optical path. A rotation of the coil produces a shift between in the respective resonance frequencies of the resonant coil and the frequency difference, such as may be measured by tuning the CW beam and CCW beam frequencies to match the resonance frequency shift of the coil due to rotation, indicates the rotation rate.
The RFOG may encounter a variety of anomalies that decrease the accuracy of the rotational rate measurement. In a reflecting mode, the ring resonator reflects light having a state matched with a pre-determined state of the resonator, and the resonance frequencies for each of the CW and CCW paths through the fiber optic coil are detected by monitoring the light that does not enter the resonator. The resonance is thus observed as a “resonance dip” because less light is observed when the resonator is near resonance than when the resonator is not near resonance. As previously mentioned, successive recirculation of each of the counter-propagating light beams produces constructive interference at the resonance frequencies, and the center of a resonance dip in the resonance lineshape indicates a resonance frequency. It is desirable to have a definitive symmetrical resonance dip to more accurately indicate the resonance frequency. To this end, the resonator may be designed to circulate light in a pre-determined state (e.g., TEM00-S representing a state of the light having a lowest order spatial mode and a vertical polarization in a free space resonator).
Generally, a majority of the light having the matched state, or the desired input light component, is reflected by the resonator to circulate in the fiber optic coil. Non-resonant, stray undesired light (e.g., light that is not properly matched in the polarization mode or the spatial mode of the resonator) may interfere with the light in the matched state that is reflected by the resonator and circulated in the resonator and, thus, produce errors in the detection of the resonance centers. The resonance dip may be affected by several factors including, but not necessarily limited to, a residual launch light component in the input light beam to the resonator having either 1) an undesired polarization state or 2) light, from the input light beam to the resonator, with a spatial distribution that overlaps with higher order spatial modes of the light in the resonator. Both of these are due to an imperfect input light condition or launch condition at the input to the resonator. Although the residual launch light component may not resonate in the resonator when the desired light component is near resonance, this residual light may adversely affect the observed shape of the resonance dip resulting from the desired light component.
In addition to the interference from non-resonant residual light components in the launch condition, higher order spatial modes of light in the resonator may resonate or be near resonance and may alter the shape of the resonance dip for the mode used for rotation sensing. For example, resonance of the higher order spatial modes of light may produce additional dips close to the resonance lineshape of the desired mode used for rotation sensing. Additionally, the second polarization state may also resonate or be near resonance and may alter the shape of the resonance dip for the other polarization mode used for rotation sensing. When these additional dips are positioned in proximity to the resonance dips associated with the resonance frequency or superimposed onto the resonance dips associated with the resonance frequency, the shape of the resonance dip associated with the resonance frequency may be altered. As previously mentioned, without exciting a resonance, input light that is not properly matched in the polarization mode or the spatial mode of the resonator may distort the shape of the resonance dip of the mode used for rotation sensing.
Interference from non-mode matched residual light in the launch condition having either the undesired polarization state or higher order spatial mode components of the resonator may complicate identification of the resonance centers and provide inaccurate determinations of resonance frequencies and rotations rates. Determination of the resonance centers for each of the resonance frequencies of the CW and CCW beams directly affects the rotational rate measurement and, thereby severely limits the accuracy of the RFOG.
Several mechanisms may couple light into the undesired polarization state of the fiber optic resonator. In general, light traveling in the undesired polarization state results from a combination of these mechanisms. As previously mentioned, light may be cross-coupled inside the recirculating device, such as a fiber coupler. Light may also excite the second polarization state, or couple into the second polarization state, of the resonator when undesirably injected into the optical fiber with a component of the light in the undesired polarization state. This may be exacerbated by possible variances in the states of polarization of the fiber inside the resonator due to temperature or stress variation, thereby making repeated light launches into one polarization state of the resonator more difficult. Even if the light beams are originally introduced to the coil of the RFOG in the first polarization mode, the optical fiber may have one or more imperfections that couple light into the second polarization mode.
One way of limiting such cross-talk between polarization modes of the fiber resonator is to employ polarization preserving fiber. Polarization preserving fiber incorporates stresses defining different speeds of light (i.e., birefringence) that attenuate the cross-coupling of light from one polarization axis of the fiber to the other. This feature of polarization preserving fiber stabilizes the polarization mode of the ring resonator, thereby assisting the task of stably launching a fraction of light into a desired mode. Using conventional optical fibers, particularly polarization preserving fibers, the difference in the speed of light between light traveling on the two principle axes of polarization in the fiber typically varies with temperature. This variation can cause the relative resonance frequencies of the two polarization states to vary with temperature. In some instances, the resonance frequency of the undesired polarization state may coincide with the resonance frequency of the desired polarization state under some environmental conditions.
Polarization-induced errors may severely limit the accuracy of the RFOG because the accuracy of the determination of the resonance centers, and thus the resonance frequencies in the CW and CCW directions, directly affects the rotational rate measurement. Additionally, these errors in the measurement may change radically with respect to the temperature in conventional optical fibers due to the sensitivity of the associated birefringence to temperature. Consequently, the gyro output may drift without influence from a variation in rotation rate. Additional error mechanisms in an RFOG employing conventional glass fibers that are attributable to the propagation of light in the solid glass medium of the optical fiber include optical Kerr Effect, Stimulated Brillouin Scattering, and Raleigh back-scattering.
Accordingly, it is desirable to provide a fiber optic gyro having a more accurate rotational rate measurement by minimizing the interference from input light matched to the undesired polarization state of the resonator or input light matched to the undesired spatial modes of the resonator. More particularly, it is desirable to provide a fiber optic gyro having a transmission mode for detecting resonance centers of the resonator. In addition, it is desirable to provide a method for measuring a rotational rate measurement in a fiber optic gyro that minimizes interference from input light that is incorrectly or unintentionally matched to the undesired polarization state or undesired spatial modes of the resonator. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.